1. Field of the Invention
The present invention is related generally to conveyors, and more particularly to conveyors which utilize straight-running, sideflexing, or multiflexing tab chains.
2. Statement of the Prior Art
Within the conveyor art, there are three known types of chain conveyors: (1) those that support a product being conveyed; (2) those that carry an actuating element between two chains; and, (3) those that operate overhead or are set within the floor. See, e.g., Conveyor Equipment Manufacturers Association, Conveyor Terms and Definitions, 3d ed., 1979. Two basic types of chain conveyor, referred to as a "flat top plate" conveyor or a "hinged top plate" conveyor, are known for their abilities to carry bottles and jars, cans, small parts and food products over substantially horizontal, straight and curved paths. Such top plate chain conveyors usually consist of an "endless" chain of links 60, shown in FIG. 6., which is formed in a continuous loop around a drive or head sprocket and a tail or idler sprocket. The top plates 62 of each link 60 are welded on, snapped on or are formed integrally with respective links in the chain and tabs 61 extend transversely on the underside of each link 60 and are space below the top plates 62, as shown in FIGS. 6 and 7. As thus formed, the endless chains are guided through a carrying way 22 from the driven sprocket, around the idler sprocket and back to the drive sprocket through a return way. As shown in FIG. 6(b), the carrying way 22 is disposed between the top plate 62 and the tab 61. As shown in FIG. 7, at some locations, particularly where the conveyor goes around a corner, channels are attached to, and form a part of, the carrying way 22. Details of such chain conveyors are found in Manual #831270-B19C R of Rexnord Inc. which is incorporated herein by reference. See, Rex TableTop.RTM. chain Engineering Manual (1985).
Sideflexing and multiflexing chain conveyors require some method of chain retention where going around corners for preventing the possibility of the chain lifting out of its carrying way (i.e., its "track"). One known method of such retention utilizes beveled surfaces on each link in the chain for mating with a complementary beveled surface on corner wear strips on either side of the track. Another known method shown in FIGS. 6 and 7, utilizes extended hold-down tabs 61 on the under side of each link 60 in the chain. The present invention is directed to these "tab chains" which provide positive retention through the tracking of the chain, around its carrying path, between such hold down tabs and their corresponding top plates.
Dependent upon the "slippage factor" of a particular tab chain, products on the conveyor may accumulate. That is, slippage factors take into consideration the coefficient of friction of the materials used to make the tab chain, and account for accumulation of the product on the conveyor and the duration of time it occurs. Significant problems of "backline pressure" are experienced during such accumulation, however, and those of skill in the conveyor art have persistently sought out designs which avoid these problems.
One approach in conveyor design which was initially used was the use of off-line accumulators. Such approaches transferred the product that was accumulating out of the mainstream, and on to adjacent conveyor paths that subsequently reentered the mainstream at a point downstream of the accumulator. As a result of a longer path which would be taken by product in such off-line accumulators, problems of backline pressure could be avoided. However, since the off-line accumulators require an additional adjacent conveyor path, they are more difficult to design and more costly to implement.
Another approach which has been utilized which avoids any added conveyor paths is the in-line accumulator. Many low friction thermoplastic materials (e.g., LF Acetal) have been used for chains to enable the product to slide upon the moving chain surface during accumulation. Such low friction material reduced backline pressure and, thereby, reduced product damage during accumulation. However, product damage still occurred, requiring further development. "Low backline pressure" chains (e.g., LBP821 and L8P882TAB series chains manufactured by Rexnord Inc.) brought about a significant reduction in backline pressure via a series of small, closely spaced rollers. These rollers provided a stable conveying surface while at the same time provided positive motion as necessary. Nevertheless, backline pressure has heretofore never been eliminated or even substantially eliminated.